The objective of this study is to evaluate the safety and tolerability of two active doses of RGN-352 (thymosin beta 4, Tβ4, Injectable Solution) in patients with acute myocardial infarction receiving percutaneous coronary intervention angioplasty with or without stent placement. Approximately 75 subjects will be randomized to receive one of two RGN-352 doses of 1200 mg, or 450 mg, or placebo, administered iv by iv push daily for the first 3 consecutive days and weekly for 4 more weeks.